In recent years, with technical advancement in performance of electronic apparatuses such as smart phones, tablets and notebook computers, heat generation densities of heat-generating components have rapidly increased, and therefore, heat diffusion techniques have been required for these electronic apparatuses.
Particularly, small-sized mobile apparatuses frequently come into direct contact with the human body, and increases in temperatures of exterior surfaces of their housings have been serious problems. As one problem concerning increases in temperatures of exterior surfaces of the housings of the mobile apparatuses, low temperature burn injuries can be mentioned. Low temperature burn injuries are a type of burn caused by long-term exposure of the human body to a temperature higher than the body temperature. There has been a report that, when the temperature is 44° C., burn injuries will occur in 6 hours, and that the time required to result in burn injuries will be shortened to half by an increase of 1° C. Compared with usual burn injuries, in most cases of low temperature burn injuries, injured persons take long to notice the progression of the symptoms. When the injured persons eventually notice the injuries, there are many cases that they already have serious skin injuries.
Moreover, liquid crystal or organic EL displays are used for display elements of mobile apparatuses. However, since these display elements are weak to heat, and, if heat from heat-generating components transmits to the display elements, this becomes a factor that causes uneven brightness or reduced durable life of the displays. Therefore, in order to satisfy both of high performance and reduced size/thickness of mobile apparatuses, it is required that the heat transmission to display elements is effectively reduced.
Furthermore, recently, there are many cases where low temperature burn injuries occur when small-sized notebook computers are used on the knee for a long time. In a state where downsizing of apparatuses and development of their mobile forms will be increasingly progressed in the day ahead, the most important issue to keep the temperature of the surface of the apparatuses as low as possible even by 1° C.
Meanwhile, as for a method for preventing an increase in the surface temperature of an apparatus, use of a heat insulation member can be considered in order to prevent heat from a heat generating component inside a housing of the apparatus from transmitting to the housing. For example, an attempt to reduce heat transmission to the housing, thereby averaging a temperature distribution of the inside of the housing has been made by providing a heat insulation member made of a composite of a rubber and a foam product inside the housing of the apparatus (JP-A-2002-217578).
As an alternative method, a method in which a heat conductive sheet obtained by adding a heat conductive filler to a resin, a metal foil such as aluminum foil, or the like is attached to the inner surface of the housing to lower a surface temperature of the apparatus based on heat diffusion can also be considered. In addition, a heat radiation sheet such as a graphite sheet has been used as a heat diffusion material in mobile apparatuses that require downsizing and weight reduction (JP-A-61-275116). A graphite sheet has a high heat conductivity in the surface direction, and mitigates heat spots caused on surfaces of apparatuses, consequently causing effects to lower surface temperatures of the apparatuses.
Moreover, in order to protect heat-sensitive components such as electrolytic capacitors, ICs, CPUs and optical components, a method in which a composite material having a metal film laminated on one surface of a highly-oriented graphite sheet and having a heat insulation material on the other surface is disposed between a heat-generating component and a heat-sensitive component has been known (JP-A-10-126081). In this case, the composite material is disposed therein such that the metal film is allowed to face the heat generating component, and that the heat insulation material is allowed to face the heat-sensitive component. The metal film acts to reflect the heat from the heat-generating component, and the heat insulation material acts to protect the heat-sensitive component. An aluminum foil is used as the metal film, and a ceramic or resin sheet is used as the heat insulation material.
Furthermore, as an example of a method for combining a graphite sheet and a heat insulation material, a combining method that uses an intermediate of an acrylic, silicone, epoxy or polyimide resin-based adhesive layer as an adhesive material has been known (JP-A-2009-111003). When silica aerogel is used as the heat insulation material in this method, there are cases where the resin adhesive layer intrudes into pores on a scale of several tens of nanometers present in the silica aerogel, and this results in deterioration of original heat-insulating performance of the silica aerogel.